Temperature control of plasma density probe

ABSTRACT

An apparatus for measuring a plasma parameter in a plasma processing reactor comprises a probe including a dielectric tube, a coaxial cable inserted in the dielectric tube, the coaxial cable having an open antenna tip, and a plurality of spacers disposed between the coaxial cable and the dielectric tube. The plurality of spacers define a plurality of ducts through which a cooling fluid is adapted to be circulated to control a temperature of the probe.

FIELD OF THE INVENTION

The present invention relates generally to plasma, and relates specifically to measuring plasma parameters, for example plasma density, in plasma processing reactors.

BRIEF SUMMARY OF THE INVENTION

An aspect of the present invention is to provide an apparatus for measuring a plasma parameter in a plasma processing reactor. The apparatus comprises a dielectric tube, a sensor disposed in the dielectric tube and a connector disposed in the dielectric tube and coupled to the sensor. The apparatus further comprises a plurality of spacers disposed between the connector and the dielectric tube. The plurality of spacers define a plurality of ducts through which a cooling fluid is adapted to be circulated to control a temperature of the probe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus for measuring plasma density, according to an embodiment of the invention;

FIG. 2 is a transverse cross-section of the apparatus for measuring plasma density shown of FIG. 1;

FIG. 3 is a schematic representation of an apparatus for measuring a plasma parameter, according to another embodiment of the invention;

FIG. 4 is a transverse cross-section of the apparatus for measuring a plasma parameter shown in FIG. 3;

FIG. 5 is a schematic representation of a plasma apparatus incorporating the plasma parameter measuring apparatus of FIGS. 1 and 3, according to an embodiment of the present invention; and

FIG. 6 shows various temperature control feedback configurations, according to various embodiments of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Plasma is used in material processing reactors because it provides significant benefits in processing rate, accuracy, and processing capabilities in comparison with non-plasma methods. In order to better control the plasma and the processes taking place inside a plasma reactor, it is sometimes useful to introduce measurement devices inside the plasma reactor to measure various parameters of the plasma including, for example, temperature of the plasma and density of the plasma. Plasma density defines, among other things, the radical content in the processing gas and the processing speed. The plasma density is one of the process parameters that a process engineer uses to evaluate a specific plasma process. Plasma density in a processing chamber depends on many factors, including gas composition, gas pressure, flow rate, pumping speed, geometry of the chamber and configurations of various components, such as the electrodes in the chamber, and the materials of the chamber walls.

Plasma density in a processing chamber also depends on the power of ionizing sources, which is typically radio frequency (RF) power applied from various types of coils (i.e., inductively coupled plasma sources, or ICP), RF power applied to electrodes (i.e., capacitive coupled plasma sources, or CCP), microwave power, etc. In addition, plasma density in the processing chamber depends also on the rate of loss of the plasma due to, for example, direct loss to the walls, the electrodes, and various recombination and neutralization processes occurring in the plasma.

FIG. 1 is a schematic representation of an apparatus for measuring plasma density in a plasma processing reactor, according to an embodiment of the invention. The apparatus 10 for measuring plasma density comprises a probe 12. The probe 12 comprises a coaxial cable 14 having an open antenna tip 16. The coaxial cable 14 is surrounded by a dielectric tube or sheath 18.

In an embodiment of the invention, the coaxial cable 14 is a round, flexible, two-conductor cable consisting of, from the center outwards, a center wire, a dielectric layer, a braided metal mesh sleeve, and an outer shield. The shield prevents signals transmitted on the center wire from affecting nearby components and prevents external interference from affecting the signal carried by the center wire. The center wire extends out beyond the other layers to form open antenna tip 16. The antenna tip 16 can be straight or not straight (for example slightly curved).

The tip 20 of the dielectric tube 18 is located within the area where the plasma density has to be measured. The dielectric tube 18 isolates the coaxial cable 14 and the antenna tip 16 from the plasma environment and prevents direct currents from reaching the coaxial cable 14 and the antenna tip 16. The material of the dielectric tube 18 can be selected to adjust a resonant frequency for the system. The dielectric permittivity of the material of the dielectric tube 18 can thus be chosen to correspond to an expected plasma density range (e.g., quartz has a lower dielectric permittivity than ceramic materials, such as alumina Al₂O₃). In a chemically active environment, during plasma processing of a material, dielectric depositions on the dielectric tube 18 may occur. However, these chemical depositions on the dielectric tube 18 do not affect the probe data, at least until the thickness of the deposition layer becomes thick enough to be comparable with the thickness of the dielectric tube 18.

The probe 12 includes a base 22. The base 22 closes the dielectric tube 18 and allows the probe 12 to be mounted to a component 24 of the plasma apparatus. The component 24 can be, for example, a wall of the processing chamber of the plasma apparatus. A vacuum seal can be included in base 22 to seal the probe 12 to the component of plasma apparatus (e.g., a wall of the chamber of the plasma apparatus). The base 22 can be made of, for example, metal (e.g., aluminum), ceramic material, etc.

The coaxial cable 14 can be selected to be long enough so that the antenna tip 16 is located at a position remote from the base 22. In this way, the long dielectric tube can reach to a position in the plasma apparatus where the plasma takes places.

In order to measure parameters (e.g., plasma density) of the plasma. The probe 12 is inserted in a plasma environment where temperatures can reach a level which may alter the operation of the probe and may even damage the probe if the probe is left for a long period of time in the plasma environment without appropriate heat protection. This problem may be more acute depending on the shape of the probe 12. For example, a generally slender shape of the probe 12 can prevent effective heat transfer along the dielectric tube 18 towards the base 22. As a result, this may lead to an increase of the temperature of the probe, particularly at the probe end immersed in the plasma. The increasing temperatures can cause differential thermal expansions of the probe parts which can lead to erroneous readings. For example, due to differential thermal expansions of the inner coaxial cable 14 (e.g., the metal wire) and the outer coaxial cable 14 (e.g. the braided metal mesh or the dielectric layer), a length of the antenna tip 16 can change. This may lead to a change in surface wave propagation length and hence may cause a readout of apparent change in plasma properties, e.g. plasma density.

Therefore, in order to reduce the temperature of the probe 12 to a temperature at which damage of the probe does not occur and in order to maintain the temperature of the probe relatively constant, to reduce thermal expansions, a temperature control system 26 is used. The temperature control system 26 comprises two spacers 28 a and 28 b. In an embodiment of the invention, the spacers 28 a and 28 b wrap around the coaxial cable 14 in a spiral-like configuration. FIG. 2 is a transverse cross-section of the probe 12 showing the position of the spacers 28 a and 28 b relative to the coaxial cable 14 and tube or sheath 18. In an embodiment of the invention, the spacers 28 a and 28 b consist of two elongated elements configured to wrap around the coaxial cable 14. The elongated element can have any cross-section including, circular, elliptic, rectangular, etc.

The spacers 28 a and 28 b serve at least two purposes. One purpose is to maintain a space between the coaxial cable 14 and the tube 18 so as to prevent possible contact of the coaxial cable 14 with a relatively hot surface of the tube 18 when the probe 12 is inserted in a plasma environment. Another purpose is to divide the space between the coaxial cable 14 and the tube 18 into two ducts 30 a and 30 b (shown in FIG. 2).

The two ducts 30 a and 30 b allow a cooling fluid to circulate around the coaxial cable 14 and around the interior surface of tube 18. The cooling fluid circulates in duct 30 a from the base 22 of the probe 12 toward the tip 20 of the probe and returns through duct 30 b towards the base 22 of the probe 12. The ducts 30 a and 30 b formed by the spacers 28 a and 28 b are connected at the base 22 of the probe (probe mounting block), respectively, to the inlet channel 32 a and outlet channel 32 b. The cooling fluid is supplied by a cooling fluid source (not shown) through the inlet channel 32 a. The cooling fluid enters the duct 30 a through the inlet 32 a and travels, for example spirally, guided by the spacers 28 a and 28 b, towards the tip 20 of the probe 12. When the cooling fluid reaches the tip 20 of the probe 12, the fluid passes through a perforated end-piece 34 which serves to mechanically hold the spacers 28 a and 28 b to an end of coaxial cable 14 at the probe tip 20. The arrows in FIG. 1 indicate the pathway followed by the cooling fluid from the inlet channel 32 a to the tip 20 of probe 12. After passing the perforated end-piece 34, the cooling fluid returns via the duct 30 b formed by the two spacers 28 a and 28 b towards the base 22 of the probe 12 to be evacuated through outlet 32 b. The cooling fluid evacuated through outlet 32 b can be either exhausted to atmosphere or re-circulated by re-injecting it through inlet 32 a. Because the cooling fluid evacuated through outlet 32 b may be warmer than the initial cooling fluid injected through inlet 32 a (due to contact with relatively hot areas of the probe 12), the evacuated cooling fluid may be cooled again to bring its temperature to approximately the initial temperature of the cooling fluid when first injected through inlet 32 a.

During assembly, the coaxial cable 14 is mounted in the base (probe mounting block) 22. Then the spacers 28 a and 28 b are wrapped around the coaxial cable 14 and held at the end of the coaxial cable using the end-piece 34. The dielectric tube or sheath 18 is slipped over the entire assembly coaxial cable-spacers. The dielectric tube 18 is then sealed at the base (mounting block) side to prevent fluid leakage. A seal may be achieved using soldering, brazing, or various types of adhesives.

The spacers 28 a and 28 b are wrapped around the coaxial cable 14 and the diameter of the spacers 28 a and 28 b is approximately equal half of a difference between an internal diameter of the dielectric tube 18 and an external diameter of the coaxial cable 14. In this way, by slipping the tube 18 over the coaxial cable-spacers assembly, the coaxial cable 14 is substantially centered in the dielectric tube or sheath 18.

The spacers 28 a and 28 b do not require any additional sealing against either the coaxial cable 14 external surface or the dielectric tube 18 internal surface. A contact between the spacers 28 a and 28 b and the external surface of the coaxial cable 14 and the internal surface of the tube 18 forms adequate channels for the cooling fluid to be guided therethrough without additional sealing. Of course such sealing can be employed. Furthermore, even in the case where the spacers 28 a and 28 b are not in intimate contact with the coaxial cable exterior surface and the sheath interior surface, that is the ducts 28 a and 28 b are not “perfectly” isolated from each other which may result in some fluid seeping from one duct (for example 30 a) to the other (for example 30 b), the bulk of the cooling fluid will circulate from the base 22 of the probe 12 to the tip 20 of probe and returns back to the base 22 of the probe 12 and thus provide adequate temperature control.

The spacers 28 a and 28 b can be made of any suitable material including, but not limited to, metal, plastic, ceramic materials or a combination thereof. In an embodiment of the invention, the spacers 28 a and 28 b are made of a plastic material such as Polytetrafluoroethylene (PTFE), commercialized by Dupont Corporation under the trademark TEFLON. PTFE is suitable for use as a spacer material as it is resistant to high temperatures such as the temperatures reached during plasma processing. In addition, PTFE has a low friction coefficient and is reasonably malleable which allows the spacers 28 a and 28 b to be made with a slightly larger cross-section dimensions (e.g., oversize diameter) than the dimensions of the space between the coaxial cable 14 and the dielectric tube 18. This enhances sealing of the spacers against the coaxial cable 14 and interior surface of the dielectric tube 18, while the low friction coefficient of PTFE facilitates insertion of the assembly coaxial cable-spacers inside the dielectric tube 18. Hence, even when the spacers 28 a and 28 b are slightly oversized, i.e., the spacers 28 a and 28 b have a larger diameter than the dimension of the space between the interior surface of the coaxial cable and the interior surface of the sheath 18, the low friction coefficient and malleability of PTFE would facilitate mounting the assembly coaxial cable-spacers and thus prevent potential damage of the probe components.

The cooling fluid introduced through the inlet 32 a and transported through ducts 30 a and 30 b can be any suitable fluid including gases and liquids or a mixture thereof. In an embodiment of the invention, fluorinated cooling liquids such as FLUORINERT made by 3M corporation and GALDEN made by Solvay Solexis. corporation can be used. Other liquids that may be used for such application include super-cooled gases for demanding applications (for example, applications where a long probe is used to reach inside a very hot plasma), such as liquid nitrogen or liquid carbon dioxide. Generally, liquids provide a higher heat transfer coefficient than gases. However, in certain circumstances liquids may be impractical for use. For example, in the case where the probe has a relatively small cross-section dimension (e.g., small diameter) resulting in relatively narrow ducts, a high inlet liquid pressure may be needed to drive the liquid through the narrow ducts. In such cases, gas cooling may be more suitable. Examples of gases that may be used for cooling or for temperature control in the probe 12 include air, argon and helium (in general, noble gases), nitrogen, etc.

A plasma apparatus is usually provided with a source of pressurized cooling fluid in order to control a temperature or temperatures of various components of the plasma apparatus as needed. Therefore, it is also possible to tap into this readily available source of pressurized cooling fluid and use it to control the temperature of the probe 12.

In an embodiment of the invention, the apparatus for measuring plasma density 10 further comprises one or more temperature sensors 40 inserted at a number of points along the probe 12 to provide temperature signals via wires or optical fibers 63 to a feedback control system (not shown).

For example, as illustrated in FIG. 1, the temperature sensors 40 can be attached along the coaxial cable 14. The temperature sensors 40 may be placed at regular or irregular intervals along the coaxial cable 14 so as to allow measuring of the temperature of the probe 12 at various positions along the path of cooling fluid. The signal carrying the temperature measurement can be provided as a feedback signal to the temperature control system 26 so as to control the pressure of the cooling fluid at the inlet 32 a, the flow rate and/or the inlet temperature of the cooling fluid.

FIG. 6 shows various temperature control feedback configurations, according to various embodiments of the invention. The feedback signal 601 is provided to control feedback system 600. The feedback signal 601 can be provided to a valve controller (VC) 602 which, for example, can open, close or regulate a valve 604 thus allowing controlling the flow rate of cooling fluid at the inlet 32 a. The signal feedback may also be provided to a pressure control system (PC) 606 so as to control the pressure of the cooling fluid at the inlet 32 a. Furthermore, the signal feedback may further be provided to a cooling system 608 via cooling controller (CC) 610 so as to decrease the temperature of the cooling fluid before introduction through the inlet 32 a, for example at a reservoir or chiller 612 containing the cooling fluid. Although these different control configurations are described herein as operating together, it must be appreciated that one or more temperature control configurations can be used for controlling the temperature of the probe 12. Furthermore, it must be appreciated that other temperature control configurations, not specifically discussed herein, are also within the scope of the present invention.

By providing a temperature signal feedback to the temperature control apparatus, the temperature of the probe 12 can be controlled. For example, the temperature can be maintained below a certain limit so as to prevent differential thermal expansions of the probe parts which can lead to erroneous reading of the plasma density or in certain circumstances may lead to damage of the probe. In some applications, it may be desirable to maintain the temperature within a certain range of temperatures so as to prevent temperature fluctuations and thus provide a more stable reading of the plasma parameters, e.g., plasma density.

FIG. 3 shows an apparatus for measuring a plasma parameter, according to another embodiment of the invention. The apparatus 50 comprises a sensor 52. The sensor 52 is coupled to connector 54 having an outer surface 56. The sensor 52 can be any sensor for measuring a plasma parameter such as, for example, a temperature sensor, an RF sensor, a magnetic sensor, etc. The sensor can be an electrical sensor in which case the connector 54 comprises an electrical cable or a fiber optic sensor in which case the connector 54 comprises an optical fiber. The connector 54 is surrounded by a dielectric tube 58. The dielectric tube 58 isolates the connector 54 and the sensor 52 from the plasma environment and prevents direct currents from reaching the connector 54 and the sensor 52.

The apparatus 50 further includes a base or feed block 60. The base 60 closes the dielectric tube 58 and allows the connector 54 with sensor 52 to be mounted to a component 62 of the plasma apparatus (not shown). The base 60 can be made of, for example, metal (e.g. aluminum), dielectric material or the like. The connector 54 can be selected to be long enough so that the sensor 52 disposed at an end of the connector 54 is located at a position remote from the base 60. In this way, the long dielectric. tube can reach to a position in the plasma apparatus where the plasma takes places. This allows for a more accurate measurement of the plasma parameter while at the same time preventing overheating of the base 60.

Similarly, to the embodiment shown in FIG. 1, the apparatus for measuring a plasma parameter further comprises a temperature control system 64. The temperature control system 64 can be similar to the temperature control system 26 described above. Therefore, for simplification purposes the details of the temperature control system are not repeated and only the main features are discussed. The temperature control system 64 includes spacers 66 a and 66 b which are connected at the mounting block 60 to inlet channel 68 a and outlet channel 68 b. The spacers divide the space between the outer surface 56 of connector 54 and the tube 58 into two ducts 70 a and 70 b (shown in FIG. 4). The spacers 66 a and 66 b can be wrapped around the outer surface 56 of connector 54, for example, in a spiral configuration. In an embodiment of the invention, the spacers 66 a and 66 b consist of two elongated elements configured to wrap around outer surface 56 of connector 54. The elongated elements can have any cross-section including, circular, elliptic, rectangular, etc. The spacers may be made from any suitable material including metal, plastic (such as PTFE), ceramic material or a combination thereof.

The two ducts 70 a and 70 b allow a cooling fluid to circulate around the connector 54 and around an interior surface of tube 58. The cooling fluid circulates in duct 70 a from the base 60 toward tip 72 of tube 58 and return through duct 30 b from the tip 72 to the base 60. The ducts 30 a and 30 b formed by the spacers 28 a and 28 b are connected at the base 60 to the inlet channel 68 a and outlet channel 68 b. The cooling fluid enters the duct 70 a through the inlet 68 a and travels, for example spirally, guided by the spacers 66 a and 66 b, towards the tip 72. When the cooling fluid reaches the tip 72, the fluid passes through a perforated end-piece 74 which serves to mechanically hold the spacers 66 a and 66 b to the outer surface 56 of connector 54 at the tip 72. After passing the perforated end-piece 74, the cooling fluid returns via the duct 70 b formed by the two spacers 66 a and 66 b to be evacuated through outlet 68 b.

In addition, similarly to the previous embodiments, the apparatus for measuring plasma parameter may further include one or more temperature sensors 76 inserted at a number of points along the connector 54, for example attached to outer surface 56 of connector 54, to provide temperature signals via wires or optical fibers 63 to a feedback control system 600 (shown in FIG. 6).

FIG. 5 illustrates schematically a plasma apparatus (reactor) according to an embodiment of the present invention. The plasma apparatus 90 comprises a chamber 92 having a wall 94 configured to house a substrate 96. The substrate 96 is supported by chuck 98. The plasma apparatus 90 further includes a source of gas 99 connected to the chamber 92 for introducing gas into chamber 92 and a radio-frequency source 100 configured to ionize the gas to generate a plasma P. The radio-frequency source 100 is connected to an electrode 102 (in the case of capacitive coupled plasma (CCP) source). The plasma apparatus 90 further includes a plasma parameter measuring apparatus 10, 50 attached to wall 94 of chamber 92. The plasma parameter measuring apparatus is illustrated in FIGS. 1-4 and described in detail in the above paragraphs. Although the plasma apparatus illustrated in FIG. 5 is a CCP type plasma apparatus, it must be appreciated that any kind of plasma apparatus is contemplated herein and hence falls within the scope of the present invention.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the present invention. In fact, after reading the above description, it will be apparent to one skilled in the relevant art(s) how to implement the invention in alternative embodiments. Thus, the present invention should not be limited by any of the above-described exemplary embodiments.

Moreover, the method and apparatus of the present invention, like related apparatus and methods used in the plasma arts are complex in nature, are often best practiced by empirically determining the appropriate values of the operating parameters, or by conducting computer simulations to arrive at best design for a given application. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the invention.

In addition, it should be understood that the figures, are presented for example purposes only. The architecture of the present invention is sufficiently flexible and configurable, such that it may be utilized in ways other than that shown in the accompanying figures.

Further, the purpose of the Abstract of the Disclosure is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract of the Disclosure is not intended to be limiting as to the scope of the present invention in any way. 

1. An apparatus for measuring a plasma parameter in a plasma processing: reactor, comprising: a probe including a dielectric tube, a coaxial cable inserted in the dielectric tube, the coaxial cable having an open antenna tip, and a plurality of spacers disposed between the coaxial cable and the dielectric tube, wherein said plurality of spacers define a plurality of ducts through which a cooling fluid is adapted to be circulated to control a temperature of the probe.
 2. The apparatus according to claim 1, wherein the dielectric tube is configured to isolate the coaxial cable and the antenna tip from the plasma.
 3. The apparatus according to claim 1, wherein a dielectric permittivity of a dielectric material of the dielectric tube is selected to correspond to an expected plasma density range.
 4. The apparatus according to claim 3, wherein the dielectric material comprises quartz, ceramic or a combination thereof.
 5. The apparatus according to claim 1, wherein the plurality of spacers are disposed in a spiral configuration around the coaxial cable.
 6. The apparatus according to claim 1, wherein the plurality of spacers maintain a space between the coaxial cable and the tube.
 7. The apparatus according to claim 6, wherein the spacers substantially center the coaxial cable inside the tube.
 8. The apparatus according to claim 7, wherein said space is substantially constant.
 9. The apparatus according to claim 6, wherein a diameter of the plurality of spacers is equal approximately half of a difference between an internal diameter of the tube and an external diameter of the coaxial cable.
 10. The apparatus according to claim 1, wherein the cooling fluid is adapted to be circulated through a first duct in the plurality of ducts from a first end of the tube to a second end of the tube and through a second duct in the plurality of ducts from the second end of the tube to the first end of the tube.
 11. The apparatus according to claim 10, wherein the first duct is connected to a cooling fluid inlet and the second duct is connected to a cooling fluid outlet.
 12. The apparatus according to claim 1, wherein the probe further comprises a base and the plurality of spacers are connected to the base.
 13. The apparatus according to claim 1, further comprising a perforated end-piece disposed at an end of the probe, wherein the end-piece is configured to hold the plurality of spacers to the coaxial cable at the end of the probe.
 14. The apparatus according to claim 1, wherein each spacer in the plurality of spacers consists of an elongated element.
 15. The apparatus according to claim 14, wherein the elongated element includes plastic, metal, or ceramic materials, or any combination of two or more thereof.
 16. The apparatus according to claim 15, wherein the material is polytetrafluoroethylene.
 17. The apparatus according to claim 1, wherein the cooling fluid is a liquid or a gas.
 18. The apparatus according to claim 1, wherein the cooling fluid includes a fluorinated cooling liquid, liquid nitrogen, liquid carbon dioxide, air, argon, helium, nitrogen gas, or any combination of two or more thereof.
 19. The apparatus according to claim 1, further comprising a temperature sensor disposed in the probe.
 20. The apparatus according to claim 19, wherein the temperature sensor is attached to the coaxial cable.
 21. The apparatus according to claim 19, wherein the temperature sensor is configured to measure the temperature of the probe.
 22. The apparatus according to claim 19, wherein the temperature sensor provides a temperature signal, said temperature signal being used as a feedback signal to control pressure, flow rate or temperature of the cooling fluid, or any combination of two or more thereof.
 23. The apparatus according to claim 22, further comprising a valve controller, wherein said feedback signal is provided to said valve controller so as to control the flow rate of the cooling fluid.
 24. The apparatus according to claim 22, further comprising a pressure control system, wherein said feedback signal is provided to said pressure control system so as to control the pressure of the cooling fluid.
 25. The apparatus according to claim 22, further comprising a cooling system, wherein said feedback signal is provided to said cooling system so as to increase or decrease a temperature of the cooling fluid.
 26. An apparatus for measuring a plasma parameter in a plasma processing reactor, comprising: a dielectric tube; a sensor disposed in the dielectric tube; a connector disposed in the dielectric tube and coupled to the sensor; and a plurality of spacers disposed between the connector and the dielectric tube, wherein said plurality of spacers define a plurality of ducts through which a cooling fluid is adapted to be circulated to control a temperature of the probe.
 27. The apparatus according to claim 26, wherein the dielectric tube is configured to isolate the connector and the sensor from the plasma.
 28. The apparatus according to claim 26, wherein the dielectric material comprises quartz, ceramic or a combination thereof.
 29. The apparatus according to claim 26, wherein the plurality of spacers are disposed in a spiral configuration around the connector.
 30. The apparatus according to claim 26, wherein the plurality of spacers maintain a space between the connector and the tube.
 31. The apparatus according to claim 30, wherein the spacers substantially center the connector inside the tube.
 32. The apparatus according to claim 30, wherein said space is substantially constant.
 33. The apparatus according to claim 30, wherein a diameter of the plurality of spacers is equal approximately half of a difference between an internal diameter of the tube and an external diameter of the connector.
 34. The apparatus according to claim 26, wherein the cooling fluid is adapted to circulate through a first duct in the plurality of ducts from a first end of the tube to a second end of the tube and is adapted to circulate back through a second duct in the plurality of ducts from the second end of the tube to the first end of the tube.
 35. The apparatus according to claim 34, wherein the first duct is connected to a cooling fluid inlet channel and the second duct is connected to a cooling fluid outlet channel.
 36. The apparatus according to claim 26, wherein the probe further comprises a base and the plurality of spacers are connected to the base.
 37. The apparatus according to claim 26, further comprising a perforated end-piece disposed at an end of the connector, wherein the end-piece is configured to hold the plurality of spacers at the end of the connector.
 38. The apparatus according to claim 26, wherein each spacer in the plurality of spacers consists of an elongated element.
 39. The apparatus according to claim 38, wherein the elongated element includes plastic, metal, or ceramic materials, or any combination of two or more thereof.
 40. The apparatus according to claim 39, wherein the material is polytetrafluoroethylene.
 41. The apparatus according to claim 26, wherein the cooling fluid is a liquid or a gas.
 42. The apparatus according to claim 26, wherein the cooling fluid includes a fluorinated cooling liquid, liquid nitrogen, liquid carbon dioxide, air, argon, helium, nitrogen gas, or any combination of two or more thereof.
 43. The apparatus according to claim 26, further comprising a temperature sensor disposed in the probe.
 44. The apparatus according to claim 43, wherein the temperature sensor is attached to a surface of the connector.
 45. The apparatus according to claim 43, wherein the temperature sensor is configured to measure the temperature of the cooling fluid.
 46. The apparatus according to claim 43, wherein the temperature sensor provides a temperature signal, said temperature signal being used as a feedback signal to control pressure, flow rate or a temperature of the cooling fluid, or any combination of two or more thereof.
 47. A plasma apparatus comprising: a chamber having a wall configured to house a substrate; a source of gas connected to said chamber; a plasma source; and a plasma parameter measuring apparatus, comprising: a probe including a dielectric tube, a coaxial cable inserted in the dielectric tube, the coaxial cable having an open antenna tip, and a plurality of spacers disposed between the coaxial cable and the dielectric tube, said plurality of spacers defining a plurality of ducts through which a cooling fluid is adapted to be circulated to control a temperature of the probe, wherein said plasma parameter measuring apparatus is configured to measure a density of said plasma.
 48. A plasma apparatus comprising: a chamber having a wall configured to house a substrate; a source of gas connected to said chamber; a plasma source; and a plasma parameter measuring apparatus, comprising: a dielectric tube; a sensor disposed in the dielectric tube; a connector disposed in the dielectric tube and coupled to the. sensor; and a plurality of spacers disposed between the connector and the dielectric tube, said plurality of spacers define a plurality of ducts through which a cooling fluid is adapted to circulate to control a temperature of the probe, wherein said plasma parameter measuring apparatus is configured to measure a parameter of said plasma. 